New Mexico Mineral Symposium — Abstracts

Fabric mapping, petrographic and geochemical studies in the Rattlesnake hills (RH) and Pedernal Hills (PH) have shown that two different tectono-stratigraphic assemblages may be present. The RH comprise an area of scattered exposure of approximately 50 km² of medium grade quartzo-feldspathic augen gneisses and subordinate mylonite gneisses, that are consistently foliated SSE with moderate to steep dips. Small sub-vertical aplite veins trend SSE along with the axial planes of en echelon, ptygmatic quartz veins. Originally mapped by Kelley (1972) as schists and granite gneisses, the RH appear to be lithologically distinct from the quartz-muiscovite-staurolite and quartz-muscovite¬kyanite schists, alkali granites, amphibolites and quartzites, that are foliated parallel to layering ENE, of approximately 300 km² extent that for the PH. Also, the minor hematite-malachite¬chalcopyrite mineralization, characteristic of portions of the PH, was not found in the RH. However, the RH are assumed contemporaneous to the 1.4 b.y. age of Mukhopadhyay et al.(1975) from the metamorphic rocks of the PH.

Additional tectonic analysis of Precambrian structures shows the presence of shallow (15-200) easterly thrust in the northern portion of the PH. The kinematics of this event are currently highly speculative, although field evidence suggests that these shallow thrusts post-date the high angle, north trending thrusts of Laramide age. K-Ar dating in progress on a previously unmapped, fresh, unsheared intrusive body of basalt in the RH gave an age of 848 ± 42 m.y. which helps to further constrain the structural deformational history of the RH-PH Precambrian terrain.